Caractérisation pétrologique et géotechnique des latérites de la région du Sud-Ouest Cameroun.

Abstract

The present work aims at assessing the variability of the genetical and geotechnical properties of laterites developed on different parent rocks, and to establish a CBR model from physical parameters for their use in road construction. The samples were run for petrological, geotechnical and statistical studies. After a macroscopy and microscopy description of the materials, the mineralogy was determined by optical microscopy and X-ray diffractometry. Xray fluorescence was used to determine their major elements geochemistry. The ferrous iron (FeO) was determined by titrimetry. Geotechnical studies focused on identification and characterization tests on laterites and, mechanical tests on volcanic rocks. The volcanic rocks studied are basalts with a porphiric microlitic texture, made up of olivine, pyroxene and feldspar. The sedimentary rock is a shale composed of quartz, albite, calcite and clinochlore. The basalt derived laterites (LRB) are red to dark red and the profiles are tchicker than those developed on shale derived laterites (LRS) and on gneisses derived laterites (LRG). The LRS and LRG materials are yellow to yellowish color. The LRB materials are ferrugino-silicoaluminous and more affected than the LRA silico-ferrugino-aluminous materials. The LRA materials are kaolinitized to moderately lateritized, while the LRB materials are moderately to strongly lateritized. The Principal Component Analysis shows 03 groups of geotechnical parameters: (1) The group of granulometrical parameters; (2) the group of plasticity parameters and, (3) the group of California bearing ratio (CBR), activity ratio (Ac), gravel and grading modulus (Gm) parameters. Three types of laterites are present in the area : (1) class 1/3 laterites, which are the SRL materials, characterized by maximum dry density (MDD), sand and Gm parameters ; (2) class 2/3 laterites, associated to the GRL materials, characterized by Gm, gravel, optimum moisture contents (OMC) and Ac parameters and, (3) class 3/3 laterites, made of LRB materials, characterized by plasticity modulus (Pm), swelling potential (εs), plasticity plasticity (Pp), plasticity index (PI), liquid limit (LL), mortar, skeleton, plasticity limit (PL), OMC, Fines and clays parameters. The best adjustment of the CBR by the parameters, Fines, PI, OMC and MDD is given by the equation: CBR = - 0.0616 (˂ 80µm) + 0.0892PI – 0.1035OMC + 0.9347MDD + β (constant) with R2 = 0.95. The mean values of the micro-deval (MDE) coefficient of basalt aggregates, lower than 13%, coupled with the mean values of the Los Angeles (LA) coefficient, lower than 30%, suggest their used as base layer and wearing course. LRB materials are less dense, slightly Fines, more plastic and with lower CBR values than LRA and LRG materials. The mean values of CBR (30–70%), MDD (> 1.80), Pm (> 250), Gm (<0.3 x 106 esa), PI (30–20%) and Fines (35–30%) parameters, coupled with mean day traffic on national roads, show that the LRA and LRG materials can be used as subbase layers for any type of traffic and, as base layers for low volume traffic. LRB materials are used as subbases for any type of traffic.